Semiconductor element drive device, power conversion device, motor drive device, semiconductor element drive method, power conversion method, and motor drive method

ABSTRACT

The present invention provides a high-reliability IGBT drive device that is capable of avoiding an accident such as an upper/lower arm short circuit when a malfunction occurs due to a potential change at a connection point between series-connected IGBTs. High- and low-voltage side IGBTs are complementarily ON/OFF controlled before and after dead time. A reset pulse that turns OFF the high-voltage side IGBT is generated during the dead time as described in the following example. The reset pulse is generated (1) immediately before an ON instruction for the low-voltage side IGBT, (2) in such a manner as to have a period that begins immediately before the ON instruction for the low-voltage side IGBT and overlaps with the ON instruction, (3) continuously during the dead time, (4) continuously during dead time immediately before the low-voltage side IGBT turns ON, or (5) in such a manner as to invalidate the ON instruction for the low-voltage side IGBT when an ON state of the high-voltage side IGBT is observed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor element drive device, apower conversion device having the semiconductor element drive device, amotor drive device, a semiconductor element drive method, a powerconversion method, and a motor drive method.

2. Description of the Related Art

In a power conversion device in which high- and low-voltage sidesemiconductor elements are series-connected between main power supplyterminals to form high- and low-voltage side arms, the high-voltage sidesemiconductor element is driven at a floating potential. Therefore, adrive circuit for the high-voltage side semiconductor element uses aninsulated power supply. Further, a level shift circuit is necessarybecause a low-voltage side circuits transmits a drive signal to ahigh-voltage side circuit. The level shift circuit generally includes apulse generation circuit and two nMOS-FETs. The pulse generation circuitgenerates a set pulse and reset pulse from the drive signal. Thegenerated set pulse and reset pulse become gate inputs for thenMOS-FETs. It is demanded that this type of power conversion deviceexhibit, for instance, high withstand voltage, low loss, and highreliability.

In the power conversion device, the potential of a connection pointbetween the low- and high-voltage side semiconductor elements suddenlychanges from a ground potential of the low-voltage side semiconductorelement to a main power supply voltage. In this instance, a parasiticcapacitance exists between a drain and a source of the nMOS-FETs, whichconstitute the level shift circuit. Therefore, a current simultaneouslyflows to the two nMOS-FETs, which constitute the level shift circuit,due to a drastic potential change (dV/dt=great). The flow of such acurrent may transmit a wrong signal to a high-voltage side controlcircuit, thereby causing the high-voltage side semiconductor element toerroneously turn ON/OFF.

JP-A-1997-172366 and JP-A-2005-304113 call a drastic potential change ata series connection point a self-excited or separately-excited dV/dt,and disclose measures for preventing the level shift circuit frommalfunctioning due to the self-excited or separately-excited dV/dt. Morespecifically, JP-A-1997-172366 avoids malfunction by providing thehigh-voltage side control circuit with a filter circuit, whereasJP-A-2005-304113 integrates the difference between a set signal and areset signal and transmits a control signal to avoid malfunction.

SUMMARY OF THE INVENTION

As disclosed in JP-A-1997-172366 and JP-A-2005-304113, a filter or thelike is used to avoid malfunction that may occur due to great dV/dt. Ina power conversion device, however, various types of great dV/dt arise.Further, as the withstand voltage and output of the power conversiondevice are increased, a wider variety of dV/dt tend to occur. Forexample, high dV/dt, long-duration dV/dt, and high-frequency vibrationoccur. Therefore, the aforementioned semiconductor element drive deviceconfiguration cannot completely avoid malfunction due to dV/dt. Inaddition, the aforementioned configuration cannot correct a malfunctionif it should occur.

An object of the present invention is to increase the dV/dt resistanceof a semiconductor element drive device.

Another object of the present invention is to provide a semiconductorelement drive device that is capable of preventing serious accidentssuch as an upper/lower arm short circuit in the event of a malfunction,a power conversion device having the semiconductor element drive device,and a motor drive device.

Still another object of the present invention is to provide asemiconductor element drive method for increasing the dV/dt resistanceof the semiconductor element drive device and preventing seriousaccidents such as an upper/lower arm short circuit in the event of amalfunction, a power conversion method for use with the semiconductorelement drive method, and a motor drive method.

According to an aspect of the present invention, there is provided asemiconductor element drive device or method for driving a low-voltageside semiconductor element, which is one of two semiconductor elementsthat are series-connected between main power supply terminals, withreference to a ground potential of the low-voltage side semiconductorelement; driving a high-voltage side semiconductor element by ahigh-voltage side drive circuit that is referenced to a potential of aseries connection point between the semiconductor elements; generating aset pulse signal and a reset pulse signal, which serve as an ONinstruction and an OFF instruction for the high-voltage sidesemiconductor element, with reference to the ground potential of thelow-voltage side semiconductor element; shifting the levels of the setpulse signal and reset pulse signal toward a high-voltage side that isreferenced to the potential of the series connection point andtransmitting the resulting set pulse signal and reset pulse signal tothe high-voltage side drive circuit; turning ON/OFF the twosemiconductor elements complementarily before and after dead time duringwhich the semiconductor elements are both OFF; wherein the reset pulsesignal is generated during the dead time.

According to another aspect of the present invention, there is providedthe semiconductor element drive device or method, wherein a reset pulseis generated immediately before an ON instruction for the low-voltageside semiconductor element.

According to another aspect of the present invention, there is providedthe semiconductor element drive device or method, wherein a reset pulseis generated that begins immediately before the ON instruction for thelow-voltage side semiconductor element and overlaps with the ONinstruction for the low-voltage side semiconductor element.

According to another aspect of the present invention, there is providedthe semiconductor element drive device or method, wherein a reset pulseis continuously output during the dead time during which the high- andlow-voltage side semiconductor elements are both OFF.

According to another aspect of the present invention, there is providedthe semiconductor element drive device or method, wherein a reset pulseis continuously output during the dead time during which the high- andlow-voltage side semiconductor elements are both OFF immediately beforethe low-voltage side semiconductor element turns ON.

According to still another aspect of the present invention, there isprovided the semiconductor element drive device or method, wherein theON instruction for the low-voltage side semiconductor element isinvalidated when an ON state of the high-voltage side semiconductorelement is observed.

Preferred embodiments of the present invention provide a semiconductorelement drive device and method, a power conversion device and method,or a motor drive device and method for increasing the dV/dt resistanceof a series connection point between semiconductor elements, preventingaccidents such as an upper/lower arm short circuit, and offering highreliability and high withstand voltage.

Other objects and features of the present invention will be apparentfrom the detailed description of the preferred embodiments whichfollows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the overall configuration of a motor drive device that usesa power semiconductor element drive device according to a firstembodiment of the present invention;

FIG. 2 is a level shift operation timing diagram according to the firstembodiment of the present invention;

FIG. 3 is a level shift operation timing diagram according to a secondembodiment of the present invention;

FIG. 4 is a level shift operation timing diagram according to a thirdembodiment of the present invention;

FIG. 5 is a level shift operation timing diagram according to a fourthembodiment of the present invention;

FIG. 6 is a level shift operation timing diagram according to a fifthembodiment of the present invention;

FIG. 7 shows the overall configuration of a motor drive device that usesa power semiconductor element drive circuit according to a sixthembodiment of the present invention; and

FIG. 8 is a level shift operation timing diagram according to a sixthembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will now be described in detailwith reference to the accompanying drawings.

First Embodiment

FIG. 1 illustrates the overall configuration of a motor drive devicethat uses a power semiconductor element drive device according to afirst embodiment of the present invention. In reality, a common deviceconfiguration is such that three-phase AC power is output to supplypower to a three-phase induction motor or other load. For the sake ofexplanation, however, the figure shows only one phase.

An IGBT (insulated gate bipolar transistor) 3, which is a high-voltageside semiconductor element, and an IGBT 4, which is a low-voltage sidesemiconductor element, are series-connected to both ends of a main powersupply 1 and a smoothing capacitor 2. Free-wheel diodes 5, 6 areconnected in inverse parallel with the IGBTs 3, 4, respectively. Aseries connection point 7 between the IGBTs 3, 4 is an AC power outputpoint to which a load (e.g., three-phase induction motor) 8 isconnected.

Although the present embodiment uses the IGBTs as semiconductorswitching elements, MOS-FETs may be alternatively used. If the MOS-FETsare used, the free wheel diodes 5 and 6 need not be used.

The low-voltage side IGBT 4 operates with reference to a groundpotential 9. A power supply 11 supplies power to a drive circuit 10 forthe low-voltage side IGBT. Although the drive circuit 10 often includesa protection circuit, it is merely referred to as the drive circuit.

An insulated power supply 15 supplies power to a drive circuit 12 fordriving the high-voltage side IGBT 3, an RS flip-flop 13, and a logicfilter 14. As is the case with the drive circuit 10, the drive circuit12 often includes a protection circuit. However, it is merely referredto as the drive circuit. The logic filter 14 is a circuit that shuts offsignals when it receives both a set pulse signal and a reset pulsesignal. This circuit prevents a malfunction that may occur due, forinstance, to a voltage change dV/dt at the series connection point 7.

A level shift circuit 16 includes an nMOS-FET 17 for set pulse signaltransmission, an nMOS-FET 18 for reset pulse signal transmission,resistors 19, 20, and zener diodes 21, 22. When a set pulse SP is outputfrom a pulse generation circuit 23, the nMOS-FET 17 conducts, causing avoltage drop across the resistor 20 and transmitting a set pulse signalto a high-voltage potential side. The set pulse signal transmitted tothe high-voltage potential side travels through the logic filter 14, RSflip-flop 13, and drive circuit 12 and turns ON the IGBT 3. When a resetpulse RP is output from the pulse generation circuit 23, the nMOS-FET 18conducts, causing a voltage drop across the resistor 19 and transmittinga reset pulse signal to the high-voltage potential side. The reset pulsesignal transmitted to the high-voltage potential side travels throughthe logic filter 14, RS flip-flop 13, and drive circuit 12 and turns OFFthe IGBT 3. The Zener diodes 21 and 22 suppress overvoltage to protectperipheral circuits.

A control section 24 outputs a control signal CS, which is an ON/OFFinstruction for the IGBTs 3, 4. When, for instance, the load 8 is athree-phase induction motor, and its speed is to be controlled, theoutput voltage and output frequency of an inverter that includes theIGBTs 3, 4 are controlled in accordance with the deviation between aspeed instruction and actual speed so that the speed of the three-phaseinduction motor approaches a value prescribed by the speed instruction.The control signal CS may be regarded as an ON/OFF instruction for theIGBTs 3, 4 that is derived from the control system described above.

A dead time generation circuit 25 receives the control signal CS fromthe control section 24, obtains dead time DT, and generates alow-voltage side drive signal LD and a high-voltage side drive signalHD.

To increase the withstand voltages and outputs of the semiconductorelement drive device, power conversion device, and motor drive device,the present embodiment uses separate silicon chips as the set pulsesignal transmission nMOS-FET 17, reset pulse signal transmissionnMOS-FET 18, low-voltage side circuit 10, and high-voltage side circuit.An MCM (multi-chip module) structure or SIP (system in package)structure is employed so that these separate silicon chips are fastenedto an insulated substrate, wired, and packaged with resin.Alternatively, however, an SOC (system on chip) structure may beemployed so that all semiconductor circuits are integrated into a singlesilicon chip.

An operation of the first embodiment will now be described withreference to FIG. 2.

FIG. 2 is a level shift operation timing diagram according to the firstembodiment of the present invention. When the control signal CS is High,it is a low-voltage side IGBT ON instruction (high-voltage side IGBT OFFinstruction) CS1, CS2 . . . When the control signal CS is Low, it is ahigh-voltage side IGBT ON instruction (low-voltage side IGBT OFFinstruction). Upon receipt of the control signal CS, the dead timegeneration circuit 25 generates the low-voltage side drive signal LD andhigh-voltage side drive signal HD, which are used to turn ON/OFF the twoIGBTs complementarily before and after dead time during which the IGBTsare both OFF. The low-voltage side drive signal LD and high-voltage sidedrive signal HD are ON instructions when they are High and OFFinstructions when they are Low. Dead times DT1 to DT4 are provided toavoid an upper/lower arm short circuit.

The low-voltage side drive signal LD directly goes to the low-voltageside drive circuit 10 and serves as a gate signal for the low-voltageside IGBT.

On the high-voltage side, on the other hand, the pulse generationcircuit 23 generates a set pulse SP and reset pulse RP for generating agate signal for the high-voltage side IGBT in accordance with thehigh-voltage side drive signal HD (including the low-voltage side drivesignal LD as needed). Consequently, the high-voltage side IGBT is turnedON by set pulses SP1 and SP2 that prevail when dead time DT2 or DT4elapses after the falling edge of control signal CS1 or CS2. Resetpulses RP1 and RP3 serve as OFF instructions for the high-voltage sideIGBT 3 that correspond to the rise of the control signal CS. In thisinstance, the rises of reset pulses RP2 and RP4 overlap with those oflow-voltage side drive signals LD1 and LD2 for a short period of time tdin consideration of turn-ON delay time.

As described above, the pulse generation circuit 23 according to thefirst embodiment includes circuit means for generating reset pulses PR2and PR4 during dead times DT1 and DT3. The circuit means generates resetpulses PR2 and RP4 immediately before the ON instructions LD1, LD2 forthe low-voltage side IGBT 4. Further, the circuit means generates resetpulses PR2 and RP4 that begin immediately before the ON instructions LD1and LD2 for the low-voltage side IGBT and overlap with the ONinstructions for the low-voltage side IGBT for period td.

Consequently, an upper/lower arm short circuit can be avoided even ifthe high-voltage side IGBT 3 erroneously turns ON during dead time DT1or DT3 due, for instance, to a potential change dV/dt at the seriesconnection point 7 between IGBTs 3 and 4.

Second Embodiment

A second embodiment of the present invention will now be described withreference to FIG. 3.

FIG. 3 is a level shift operation timing diagram according to the secondembodiment of the present invention. The second embodiment is equal tothe first embodiment in the overall configuration of the motor drivedevice based on a power IGBT drive circuit and the power conversiondevice.

The basic operation of the second embodiment is the same as that of thefirst embodiment. However, the pulse generation circuit 23 according tothe second embodiment generates reset pulses RP2 and RP4 immediatelybefore set pulses SP1 and SP2 during dead times DT2 and DT4.

Consequently, even if the high-voltage side IGBT 3 turns ON during deadtime DT2 or DT4, it can be immediately turned OFF. This makes itpossible to prevent the high-voltage side IGBT 3 from prematurelyturning ON and avoid controllability deterioration.

Third Embodiment

A third embodiment of the present invention will now be described withreference to FIG. 4.

FIG. 4 is a level shift operation timing diagram according to the thirdembodiment of the present invention. The third embodiment is equal tothe first embodiment in the overall configuration of the motor drivedevice based on a power semiconductor element drive circuit and thepower conversion device.

The basic operation of the third embodiment is the same as that of thefirst embodiment. Reset pulses RP2 and RP5 are generated immediatelybefore the low-voltage side IGBT 4 turns ON, and an OFF instruction isoutput to the high-voltage side IGBT 3.

In addition, the third embodiment generates reset pulses RP3 and RP6immediately before set pulses SP1 and SP2. This makes it possible toprevent an upper/lower arm short circuit and avoid controllabilitydeterioration even if a malfunction occurs during dead time DT1, DT2,DT3, or DT4.

Fourth Embodiment

A fourth embodiment of the present invention will now be described withreference to FIG. 5.

FIG. 5 is a level shift operation timing diagram according to the fourthembodiment of the present invention. The fourth embodiment is equal tothe first embodiment in the overall configuration of the motor drivedevice based on a power semiconductor element drive circuit and thepower conversion device.

The fourth embodiment continuously outputs reset pulses RP1 to RP4during all dead times DT1-DT4. This ensures that the logic filter 14shuts off an incorrect signal even if the level shift circuit 16malfunctions due, for instance, to a potential change dV/dt at theseries connection point 7 between IGBTs 3 and 4 during dead time DT.Consequently, it is possible to prevent an upper/lower arm short circuitand avoid controllability deterioration even if a malfunction occursduring all dead times DT1-DT4.

Fifth Embodiment

A fifth embodiment of the present invention will now be described withreference to FIG. 6.

FIG. 6 is a level shift operation timing diagram according to the fifthembodiment of the present invention. The fifth embodiment is equal tothe first embodiment in the overall configuration of the motor drivedevice based on a power semiconductor element drive circuit and thepower conversion device.

In the fifth embodiment, the pulse generation circuit 23 continuouslyoutputs reset pulses RP1 and RP2 during dead times DT1 and DT3 duringwhich the high- and low-voltage side IGBTs 3, 4 are both OFF immediatelybefore the low-voltage side IGBT 4 turns ON.

Consequently, the fifth embodiment makes it possible to prevent anupper/lower arm short circuit even if a malfunction occurs due, forinstance, to a potential change dV/dt at the series connection point 7between IGBTs 3 and 4 during dead time DT1 or DT3. Further, the fifthembodiment consumes less power than the fourth embodiment.

Sixth Embodiment

FIG. 7 illustrates the overall configuration of a motor drive devicethat uses a power semiconductor element drive circuit according to asixth embodiment of the present invention. The sixth embodiment of thepresent invention will now be described while omitting descriptions thatoverlap with those of the first embodiment.

A high-voltage side gate voltage observation circuit 26 observes thegate voltage of the high-voltage side IGBT 3 with reference to thepotential of the connection point 7 between the high-voltage side IGBT 3and low-voltage side IGBT 4. The gate voltage of the high-voltage sideIGBT 3 is then compared against a setting to judge whether thehigh-voltage side IGBT 3 is ON or OFF. A level reduction circuit 27transmits the information output from the high-voltage side gate voltageobservation circuit 26 to a low-voltage side gate voltage observationcircuit 28, which is referenced to the ground potential 9 of thelow-voltage side IGBT 4. The level reduction circuit 27 includes apMOS-FET 29, a resistor 30, and a Zener diode 31. When the high-voltageside gate voltage observation circuit 26 generates an output, thepMOS-FET 29 conducts to develop a voltage across the resistor 30 andtransmit the ON/OFF information about the high-voltage side IGBT 3 tothe low-voltage side gate voltage observation circuit 28. The Zenerdiode 31 protects peripheral circuits against overvoltage.

The information transmitted to the low-voltage side gate voltageobservation circuit 28 enters the low-voltage side drive circuit 10 andpulse generation circuit 23.

When the high-voltage side IGBT 3 is ON, the low-voltage side drivecircuit 10 shuts off the ON instruction for the low-voltage side IGBT 4for invalidation purposes. The pulse generation circuit 23 continuouslyoutputs a reset pulse RP when the high-voltage side IGBT 3 is ONalthough the high-voltage side drive signal HD is Low (OFF instructionfor the high-voltage side IGBT 3).

FIG. 8 is an operation timing diagram illustrating the semiconductorelement drive device according to the sixth embodiment of the presentinvention.

If, for instance, the high-voltage side IGBT 3 erroneously turns ON togenerate its ON state signal HO11, the low-voltage side drive circuit 10invalidates low-voltage side drive signal LD1 to shut off the ONinstruction for the low-voltage side IGBT 4 because the high-voltageside IGBT 3 is ON.

The pulse generation circuit 23 continuously outputs reset pulse RP2when the high-voltage side IGBT 3 is ON with a signal H012 generated toindicate an erroneous ON state although the high-voltage side drivesignal HD is Low (OFF instruction for the high-voltage side IGBT 3).

As described above, the sixth embodiment includes a high-voltage sideIGBT monitoring circuit 26 for monitoring the ON/OFF status of thehigh-voltage side IGBT 3 with reference to the potential of the seriesconnection point 7 between IGBTs 3 and 4, and circuit means forinvalidating ON instruction LD1 for the low-voltage IGBT 4 when thehigh-voltage side IGBT monitoring circuit 26 observes the ON state ofthe high-voltage side IGBT 3.

The sixth embodiment also includes circuit means for maintaining resetpulse RP2 when the high-voltage side IGBT monitoring circuit 26 observesthe ON state of the high-voltage side IGBT 3 to generate an erroneous ONstate signal H012 even after the drive signal HDl for the high-voltageside IGBT 3 turns OFF.

The above features make it possible to prevent an upper/lower arm shortcircuit.

The present invention can be used as a semiconductor element drivecircuit, power conversion device, or motor drive device in all fields.For example, the present invention can be applied to industrial robots,railroad devices, and household electrical appliances. The presentinvention is particularly suitable, for instance, as a vehicle-mountedsemiconductor element drive circuit, power conversion device, and motordrive device, which are expected to exhibit high reliability and highwithstand voltage.

1. A semiconductor element drive device comprising: two semiconductorelements which are series-connected between main power supply terminals;a low-voltage side drive circuit for driving a low-voltage sidesemiconductor element, which is one of said two semiconductor elements,with reference to a ground potential of the low-voltage sidesemiconductor element; a high-voltage side drive circuit for driving ahigh-voltage side semiconductor element with reference to a potential ofa series connection point between said semiconductor elements; a pulsegeneration circuit for generating a set pulse signal and a reset pulsesignal, which serve as an ON instruction and an OFF instruction for thehigh-voltage side semiconductor element, with reference to the groundpotential of the low-voltage side semiconductor element; and a levelshift circuit for shifting the levels of the set pulse signal and resetpulse signal toward a high-voltage side with reference to the potentialof the series connection point and transmitting the resulting set pulsesignal and reset pulse signal to said high-voltage side drive circuit;said two semiconductor elements being complementarily turned ON/OFFbefore and after dead time during which the semiconductor elements areboth OFF, wherein said pulse generation circuit includes circuit meansfor generating the reset pulse signal during the dead time.
 2. Asemiconductor element drive device comprising: two semiconductorelements which are series-connected between main power supply terminals;a low-voltage side drive circuit for driving a low-voltage sidesemiconductor element, which is one of said two semiconductor elements,with reference to a ground potential of the low-voltage sidesemiconductor element; a high-voltage side drive circuit for driving ahigh-voltage side semiconductor element with reference to a potential ofa series connection point between said semiconductor elements; a pulsegeneration circuit for generating a set pulse signal and a reset pulsesignal, which serve as an ON instruction and an OFF instruction for thehigh-voltage side semiconductor element, with reference to the groundpotential of the low-voltage side semiconductor element; and a levelshift circuit for shifting the levels of the set pulse signal and resetpulse signal toward a high-voltage side with reference to the potentialof the series connection point and transmitting the resulting set pulsesignal and reset pulse signal to said high-voltage side drive circuit;said two semiconductor elements being complementarily turned ON/OFFbefore and after dead time during which the semiconductor elements areboth OFF, wherein said pulse generation circuit generates a reset pulseimmediately before an ON instruction for the low-voltage sidesemiconductor element.
 3. A semiconductor element drive devicecomprising: two semiconductor elements which are series-connectedbetween main power supply terminals; a low-voltage side drive circuitfor driving a low-voltage side semiconductor element, which is one ofsaid two semiconductor elements, with reference to a ground potential ofthe low-voltage side semiconductor element; a high-voltage side drivecircuit for driving a high-voltage side semiconductor element withreference to a potential of a series connection point between saidsemiconductor elements; a pulse generation circuit for generating a setpulse signal and a reset pulse signal, which serve as an ON instructionand an OFF instruction for the high-voltage side semiconductor element,with reference to the ground potential of the low-voltage sidesemiconductor element; and a level shift circuit for shifting the levelsof the set pulse signal and reset pulse signal toward a high-voltageside with reference to the potential of the series connection point andtransmitting the resulting set pulse signal and reset pulse signal tosaid high-voltage side drive circuit; said two semiconductor elementsbeing complementarily turned ON/OFF before and after dead time duringwhich said semiconductor elements are both OFF, wherein said pulsegeneration circuit generates a reset pulse which begins immediatelybefore an ON instruction for the low-voltage side semiconductor elementand overlaps with the ON instruction for the low-voltage sidesemiconductor element.
 4. A semiconductor element drive devicecomprising: two semiconductor elements which are series-connectedbetween main power supply terminals; a low-voltage side drive circuitfor driving a low-voltage side semiconductor element, which is one ofsaid two semiconductor elements, with reference to a ground potential ofthe low-voltage side semiconductor element; a high-voltage side drivecircuit for driving a high-voltage side semiconductor element withreference to a potential of a series connection point between saidsemiconductor elements; a pulse generation circuit for generating a setpulse signal and a reset pulse signal, which serve as an ON instructionand an OFF instruction for the high-voltage side semiconductor element,with reference to the ground potential of the low-voltage sidesemiconductor element; and a level shift circuit for shifting the levelsof the set pulse signal and reset pulse signal toward a high-voltageside with reference to the potential of the series connection point andtransmitting the resulting set pulse signal and reset pulse signal tosaid high-voltage side drive circuit; said two semiconductor elementsbeing complementarily turned ON/OFF before and after dead time duringwhich the semiconductor elements are both OFF, wherein said pulsegeneration circuit generates a reset pulse immediately before the setpulse signal.
 5. A semiconductor element drive device comprising: twosemiconductor elements which are series-connected between main powersupply terminals; a low-voltage side drive circuit for driving alow-voltage side semiconductor element, which is one of said twosemiconductor elements, with reference to a ground potential of thelow-voltage side semiconductor element; a high-voltage side drivecircuit for driving a high-voltage side semiconductor element withreference to a potential of a series connection point between saidsemiconductor elements; a pulse generation circuit for generating a setpulse signal and a reset pulse signal, which serve as an ON instructionand an OFF instruction for the high-voltage side semiconductor element,with reference to the ground potential of the low-voltage sidesemiconductor element; and a level shift circuit for shifting the levelsof the set pulse signal and reset pulse signal toward a high-voltageside with reference to the potential of the series connection point andtransmitting the resulting set pulse signal and reset pulse signal tosaid high-voltage side drive circuit; said two semiconductor elementsbeing complementarily turned ON/OFF before and after dead time duringwhich the semiconductor elements are both OFF, wherein said pulsegeneration circuit continuously outputs the reset pulse signal duringthe dead time during which the high- and low-voltage side semiconductorelements are both OFF.
 6. A semiconductor element drive devicecomprising: two semiconductor elements which are series-connectedbetween main power supply terminals; a low-voltage side drive circuitfor driving a low-voltage side semiconductor element, which is one ofsaid two semiconductor elements, with reference to a ground potential ofthe low-voltage side semiconductor element; a high-voltage side drivecircuit for driving a high-voltage side semiconductor element withreference to a potential of a series connection point between saidsemiconductor elements; a pulse generation circuit for generating a setpulse signal and a reset pulse signal, which serve as an ON instructionand an OFF instruction for the high-voltage side semiconductor element,with reference to the ground potential of the low-voltage sidesemiconductor element; and a level shift circuit for shifting the levelsof the set pulse signal and reset pulse signal toward a high-voltageside with reference to the potential of the series connection point andtransmitting the resulting set pulse signal and reset pulse signal tosaid high-voltage side drive circuit; said two semiconductor elementsbeing complementarily turned ON/OFF before and after dead time duringwhich the semiconductor elements are both OFF, wherein said pulsegeneration circuit continuously outputs the reset pulse signal duringthe dead time during which the high- and low-voltage side semiconductorelements are both OFF immediately before the low-voltage sidesemiconductor element turns ON.
 7. A semiconductor element drive devicecomprising: two semiconductor elements, which are series-connectedbetween main power supply terminals; a low-voltage side drive circuitfor driving a low-voltage side semiconductor element, which is one ofthe two semiconductor elements, with reference to a ground potential ofthe low-voltage side semiconductor element; a high-voltage side drivecircuit for driving a high-voltage side semiconductor element withreference to a potential of a series connection point between thesemiconductor elements; a pulse generation circuit for generating a setpulse signal and a reset pulse signal, which serve as an ON instructionand an OFF instruction for the high-voltage side semiconductor element,with reference to the ground potential of the low-voltage sidesemiconductor element; and a level shift circuit for shifting the levelsof the set pulse signal and reset pulse signal toward a high-voltageside with reference to the potential of the series connection point andtransmitting the resulting set pulse signal and reset pulse signal tothe high-voltage side drive circuit; the two semiconductor elementsbeing complementarily turned ON/OFF before and after dead time duringwhich the semiconductor elements are both OFF, the semiconductor elementdrive device further comprising: a high-voltage side semiconductorelement monitoring circuit for monitoring an ON/OFF state of thehigh-voltage side semiconductor element with reference to the potentialof the series connection point; and circuit means for invalidating an ONinstruction for the low-voltage side semiconductor element when saidhigh-voltage side semiconductor element monitoring circuit observes theON state of the high-voltage side semiconductor element.
 8. Asemiconductor element drive device comprising: two semiconductorelements, which are series-connected between main power supplyterminals; a low-voltage side drive circuit for driving a low-voltageside semiconductor element, which is one of the two semiconductorelements, with reference to a ground potential of the low-voltage sidesemiconductor element; a high-voltage side drive circuit for driving ahigh-voltage side semiconductor element with reference to a potential ofa series connection point between the semiconductor elements; a pulsegeneration circuit for generating a set pulse signal and a reset pulsesignal, which serve as an ON instruction and an OFF instruction for thehigh-voltage side semiconductor element, with reference to the groundpotential of the low-voltage side semiconductor element; and a levelshift circuit for shifting the levels of the set pulse signal and resetpulse signal toward a high-voltage side with reference to the potentialof the series connection point and transmitting the resulting set pulsesignal and reset pulse signal to the high-voltage side drive circuit;the two semiconductor elements being complementarily turned ON/OFFbefore and after dead time during which the semiconductor elements areboth OFF, the semiconductor element drive device further comprising: ahigh-voltage side semiconductor element monitoring circuit formonitoring an ON/OFF state of the high-voltage side semiconductorelement with reference to the potential of the series connection point;and circuit means for maintaining the reset pulse signal when saidhigh-voltage side semiconductor element monitoring circuit observes theON state of the high-voltage side semiconductor element even after adrive signal for the high-voltage side semiconductor element turns OFF.9. The semiconductor element drive device according to claim 7, whereinsaid high-voltage side semiconductor element monitoring circuit includesmeans, which contains a high-voltage side gate voltage monitoringcircuit, for judging whether the high-voltage side semiconductor elementis ON when a high-voltage side gate voltage is not lower than a setting.10. The semiconductor element drive device according to claim 7, furthercomprising control section which generates a control signal for saidpulse generation circuit, which serves as ON/OFF instruction for thelow- and high-voltage side semiconductor elements, and outputs the resetpulse signal when the control signal is an OFF instruction for thehigh-voltage side semiconductor element and said high-voltage sidesemiconductor element monitoring circuit observes the ON state of thehigh-voltage side semiconductor element.
 11. A power conversion devicecomprising: a semiconductor element for power conversion; and a controlsection which includes a semiconductor element drive device for drivingsaid semiconductor element; wherein said semiconductor element drivedevice is the semiconductor element drive device according to claim 1.12. A motor drive device comprising: a power conversion device whichincludes a semiconductor element and converts DC power to AC powerhaving a variable voltage and a variable frequency; a control sectionwhich includes a semiconductor element drive device for driving thesemiconductor element; and an AC motor which is driven upon receipt ofthe supply of AC output power from said power conversion device; whereinsaid semiconductor element drive device is the semiconductor elementdrive device according to claim
 1. 13. A semiconductor element drivemethod comprising the steps of: driving a low-voltage side semiconductorelement, which is one of two semiconductor elements which areseries-connected between main power supply terminals, with reference toa ground potential of the low-voltage side semiconductor element;driving a high-voltage side semiconductor element by a high-voltage sidedrive circuit which is referenced to a potential of a series connectionpoint between the semiconductor elements; generating a set pulse signaland a reset pulse signal, which serve as an ON instruction and an OFFinstruction for the high-voltage side semiconductor element, withreference to the ground potential of the low-voltage side semiconductorelement; shifting the levels of the set pulse signal and reset pulsesignal toward a high-voltage side which is referenced to the potentialof the series connection point and transmitting the resulting set pulsesignal and reset pulse signal to the high-voltage side drive circuit;and turning ON/OFF the two semiconductor elements complementarily beforeand after dead time during which the semiconductor elements are bothOFF; wherein the reset pulse signal is generated during the dead time.14. A semiconductor element drive method comprising the steps of:driving a low-voltage side semiconductor element, which is one of twosemiconductor elements which are series-connected between main powersupply terminals, with reference to a ground potential of thelow-voltage side semiconductor element; driving a high-voltage sidesemiconductor element by a high-voltage side drive circuit which isreferenced to a potential of a series connection point between thesemiconductor elements; generating a set pulse signal and a reset pulsesignal, which serve as an ON instruction and an OFF instruction for thehigh-voltage side semiconductor element, with reference to the groundpotential of the low-voltage side semiconductor element; shifting thelevels of the set pulse signal and reset pulse signal toward ahigh-voltage side which is referenced to the potential of the seriesconnection point and transmitting the resulting set pulse signal andreset pulse signal to the high-voltage side drive circuit; and turningON/OFF the two semiconductor elements complementarily before and afterdead time during which the semiconductor elements are both OFF; whereinthe reset pulse signal is generated immediately before an ON instructionfor the low-voltage side semiconductor element.
 15. A semiconductorelement drive method comprising the steps of: driving a low-voltage sidesemiconductor element, which is one of two semiconductor elements whichare series-connected between main power supply terminals, with referenceto a ground potential of the low-voltage side semiconductor element;driving a high-voltage side semiconductor element by a high-voltage sidedrive circuit which is referenced to a potential of a series connectionpoint between the semiconductor elements; generating a set pulse signaland a reset pulse signal, which serve as an ON instruction and an OFFinstruction for the high-voltage side semiconductor element, withreference to the ground potential of the low-voltage side semiconductorelement; shifting the levels of the set pulse signal and reset pulsesignal toward a high-voltage side which is referenced to the potentialof the series connection point and transmitting the resulting set pulsesignal and reset pulse signal to the high-voltage side drive circuit;and turning ON/OFF the two semiconductor elements complementarily beforeand after dead time during which the semiconductor elements are bothOFF; wherein a reset pulse which begins immediately before an ONinstruction for the low-voltage side semiconductor element and overlapswith the ON instruction for the low-voltage side semiconductor elementis generated.
 16. A semiconductor element drive method comprising thesteps of: driving a low-voltage side semiconductor element, which is oneof two semiconductor elements which are series-connected between mainpower supply terminals, with reference to a ground potential of thelow-voltage side semiconductor element; driving a high-voltage sidesemiconductor element by a high-voltage side drive circuit which isreferenced to a potential of a series connection point between thesemiconductor elements; generating a set pulse signal and a reset pulsesignal, which serve as an ON instruction and an OFF instruction for thehigh-voltage side semiconductor element, with reference to the groundpotential of the low-voltage side semiconductor element; shifting thelevels of the set pulse signal and reset pulse signal toward ahigh-voltage side which is referenced to the potential of the seriesconnection point and transmitting the resulting set pulse signal andreset pulse signal to the high-voltage side drive circuit; and turningON/OFF the two semiconductor elements complementarily before and afterdead time during which the semiconductor elements are both OFF; whereina reset pulse is generated immediately before the set pulse signal. 17.A semiconductor element drive method comprising the steps of: driving alow-voltage side semiconductor element, which is one of twosemiconductor elements which are series-connected between main powersupply terminals, with reference to a ground potential of thelow-voltage side semiconductor element; driving a high-voltage sidesemiconductor element by a high-voltage side drive circuit which isreferenced to a potential of a series connection point between thesemiconductor elements; generating a set pulse signal and a reset pulsesignal, which serve as an ON instruction and an OFF instruction for thehigh-voltage side semiconductor element, with reference to the groundpotential of the low-voltage side semiconductor element; shifting thelevels of the set pulse signal and reset pulse signal toward ahigh-voltage side which is referenced to the potential of the seriesconnection point and transmitting the resulting set pulse signal andreset pulse signal to the high-voltage side drive circuit; and turningON/OFF the two semiconductor elements complementarily before and afterdead time during which the semiconductor elements are both OFF; whereina reset pulse is continuously output during the dead time during whichthe high- and low-voltage side semiconductor elements are both OFF. 18.A semiconductor element drive method comprising the steps of: driving alow-voltage side semiconductor element, which is one of twosemiconductor elements which are series-connected between main powersupply terminals, with reference to a ground potential of thelow-voltage side semiconductor element; driving a high-voltage sidesemiconductor element by a high-voltage side drive circuit which isreferenced to a potential of a series connection point between thesemiconductor elements; generating a set pulse signal and a reset pulsesignal, which serve as an ON instruction and an OFF instruction for thehigh-voltage side semiconductor element, with reference to the groundpotential of the low-voltage side semiconductor element; shifting thelevels of the set pulse signal and reset pulse signal toward ahigh-voltage side which is referenced to the potential of the seriesconnection point and transmitting the resulting set pulse signal andreset pulse signal to the high-voltage side drive circuit; and turningON/OFF the two semiconductor elements complementarily before and afterdead time during which the semiconductor elements are both OFF; whereina reset pulse is continuously output during the dead time during whichthe high- and low-voltage side semiconductor elements are both OFFimmediately before the low-voltage side semiconductor element turns ON.19. A semiconductor element drive method comprising the steps of:driving a low-voltage side semiconductor element, which is one of twosemiconductor elements which are series-connected between main powersupply terminals, with reference to a ground potential of thelow-voltage side semiconductor element; driving a high-voltage sidesemiconductor element by a high-voltage side drive circuit which isreferenced to a potential of a series connection point between thesemiconductor elements; generating a set pulse signal and a reset pulsesignal, which serve as an ON instruction and an OFF instruction for thehigh-voltage side semiconductor element, with reference to the groundpotential of the low-voltage side semiconductor element; shifting thelevels of the set pulse signal and reset pulse signal toward ahigh-voltage side which is referenced to the potential of the seriesconnection point and transmitting the resulting set pulse signal andreset pulse signal to the high-voltage side drive circuit; and turningON/OFF the two semiconductor elements complementarily before and afterdead time during which the semiconductor elements are both OFF; whereinan ON/OFF state of the high-voltage side semiconductor element ismonitored with reference to the potential of the series connectionpoint; and wherein an ON instruction for the low-voltage sidesemiconductor element is invalidated when an ON state of thehigh-voltage side semiconductor element is observed.
 20. A semiconductorelement drive method comprising the steps of: driving a low-voltage sidesemiconductor element, which is one of two semiconductor elements whichare series-connected between main power supply terminals, with referenceto a ground potential of the low-voltage side semiconductor element;driving a high-voltage side semiconductor element by a high-voltage sidedrive circuit which is referenced to a potential of a series connectionpoint between the semiconductor elements; generating a set pulse signaland a reset pulse signal, which serve as an ON instruction and an OFFinstruction for the high-voltage side semiconductor element, withreference to the ground potential of the low-voltage side semiconductorelement; shifting the levels of the set pulse signal and reset pulsesignal toward a high-voltage side which is referenced to the potentialof the series connection point and transmitting the resulting set pulsesignal and reset pulse signal to the high-voltage side drive circuit;and turning ON/OFF the two semiconductor elements complementarily beforeand after dead time during which the semiconductor elements are bothOFF; wherein an ON/OFF state of the high-voltage side semiconductorelement is monitored with reference to the potential of the seriesconnection point; and wherein the reset pulse signal is maintained whenthe ON state of the high-voltage side semiconductor element is observedeven after a drive signal for the high-voltage side semiconductorelement turns OFF.
 21. A power conversion method for use in a powerconversion device which includes a semiconductor element for powerconversion and a control section containing a semiconductor elementdrive device for driving the semiconductor element, wherein thesemiconductor element drive method according to claim 13 is applied tothe semiconductor element drive device.
 22. A motor drive method for usein a motor drive device which includes a power conversion device, whichcontains a semiconductor element and converts DC power to AC powerhaving a variable voltage and a variable frequency; a control section,which contains a semiconductor element drive device for driving thesemiconductor element; and an AC motor, which is driven upon receipt ofthe supply of AC output power from the power conversion device, whereinthe semiconductor element drive method according to claim 13 is appliedto the semiconductor element drive device.